Pneumococci, including those with capsular polysaccharides (PSs) of types 6A or 6B are significant human pathogens. 6A and 6B serotypes produce structurally similar but serologically distinct capsular PSs. We discovered subtypes within 6A serotype (named 6A? and 6A?) with monoclonal antibodies and discovered alleles of wciN (named wciN? and wciN?), a putative glycosyl transferase in capsule gene locus. 6A? represents the classical 6A and appears to have wciN?. Although less common than 6Aa strains, 6A? strains are found worldwide, can be pathogenic, and appear to have wciN?. Further, current pneumococcal vaccines, which contain 6B? PS, appear to elicit cross-opsonic antibodies to 6A? regularly but not to 6A?. (We labeled the 6B strains with wciN? and wciN? as 6B? and 6B?, respectively. 6B? strains have not been observed.) Since 6A? may be able to avoid vaccine-induced host immunity, 6A? strains may replace 6Aa strains with continued vaccine use with resulting reduction in vaccine efficacy. To investigate this potential, we hypothesize that serogroup 6 has two new serotypes (6A? and 6B?) associated with the wciN? allele and that the new serotypes produce new capsular PSs, are poorly covered by current pneumococcal vaccines, and may increase in prevalence with vaccine use. To examine our hypothesis, we will: (A) Study the role of the wciN? allele on the 6A? and 6B? serotypes by determining the DNA sequence of the capsule gene locus of 6A?, by replacing the wciN? allele of 6Aa with the wciN? allele of 6A?, and by elucidating the molecular structure of 6A? PS with biochemical analyses. (B) Evaluate pneumococcal vaccines for their abilities to elicit antibodies capable of opsonizing the 6A?, 6A?, 6B?, and 6B? serotypes among elderly adults and young children, and evaluate 6A? PS for its ability to elicit opsonic antibodies to the four serotypes. (C) Study the effect of pneumococcal vaccination on the 6A? subtype by determining the prevalence and evolutionary relationship of 6A? isolates obtained in various parts of the world before and after clinical use of a 7-valent conjugate vaccine. Our research interest has been the study of cross-protection induced by pneumococcal vaccines. In the last funding period, we predicted vaccines may not elicit cross-protection to some serotypes. Building on our discovering the 6A? serotype, we will investigate the effect of unrecognized serotypes on vaccine-induced cross-protection. Our study will increase the understanding of the serotype shifts and replacements that follow pneumococcal vaccination, an important national health care tool. Also, our studies of 6A? should yield an elegant model showing how an emerging pathogen can appear, spread, and react to altered host immunity.